The invention discloses a high-density die-casting forming method for aluminum alloy casing die castings, which can effectively solve the problems of the large-scale aluminum alloy die castings that the gas holes are easily formed, the leakage occurs and the like. The method has comprises the following steps of: (1) adding a technological bridge structure in a mould design, thereby quickly filling to quickly fill the far ends of the die castings; (2) adopting the specific low-temperature casing and high-pressure charging die-casting process to realize high densifying of, and then highly densifying the die castings; (3) adding the vacuum die-casting step on the die-casting mould to assist the die-casting filling process; (4) controlling low gas content of aluminum liquor, and then realizing the casting process of low-gas melt. The composite die casting has the advantages that the porosity rate is smaller less than 5%; the gas content is 1-3cm<3>/(100g Al); and the appearance quality and dimension precision are high.